This invention relates to rotary furnaces, and, more particularly, to a rotary furnace that has a lightweight, freely-supported, inner chamber with a cantilevered heater structure that permits end-loading of the furnace and tilting of the furnace body for easy unloading of the furnace contents.
Prior art processing furnaces include the "muffle" furnace type wherein an inner envelope, called the muffle is surrounded by an outer envelope, called a shell. This type of furnace provides a chamber within a chamber to contain the processing atmosphere and minimize heat losses. The thermal efficiency of muffle furnaces, however, is not very good because of heat losses through the outside shell and the necessity of driving the heat through the muffle. In addition, constant cycling, which adversely affects all types of furnaces, restricts muffle furnaces to a useful life as short as twelve months or even less.
Rotary furnaces generally include a rotatable material-holding inner drum, the contents of which are heated by an internal or external heater structure. The inner drum is generally surrounded by one or more outer drums and some form of insulation is provided to maintain a tolerable temperature level at the furnace exterior.
The contents of the furnace, through a tumbling action, are thus continually exposed to the temperature and environment of the furnace for a dynamic heat treatment. While the effectiveness and efficient operation of rotary furnaces are generally recognized, it is also apparent that such furnaces are difficult to unload quickly since the contents are usually spread throughout the furnace chamber. For example, as shown in U.S. Pat. No. 2,125,912, a rotary furnace has one opening on a curved cylindrical wall of the furnace for loading and unloading purposes. The arrangement of such opening on the cylindrical periphery makes it difficult to accomplish a quick unloading of the furnace contents.
Another approach to unloading the contents of a rotary furnace is shown in U.S. Pat. No. 3,802,847 wherein inlet and outlet hoppers are provided on the cylindrical periphery of the furnace at approximate opposite ends of such furnace. The furnace must thus be tilted to be emptied, but such tilting must be slight because the output port is at a right angle to the rotational axis of the furnace. Therefore, this arrangement also does not permit a quick unloading of the furnace contents. In addition, a special heat-resistant drive system is required since one or more drive elements engage the heated periphery of the inner member.
Several other approaches to the general construction of rotary furnaces for a variety of applications are known. These include U.S. Pat. Nos. 1,980,920, 2,041,318, 2,354,100 and 2,809,442. Reference is also made to the furnaces described in Bulletin RT-874 on "Rotating Tube Furnaces" of Harper Electric Furnace Corporation, West Drullard Avenue, Lancaster, N.Y. 14086. These are only of general interest.
Due to the high temperature requirements of many rotary furnaces, it is often necessary to employ relatively thick linings in the furnace chamber which render the furnace structure massive and non-portable. In addition, because the furnace contents usually come in direct contact with the refractory lining, contamination of the furnace contents by flaking of the refractory material is a common occurrence.
Another significant problem with rotary furnaces is that the inner member usually expands and contracts at a rate different from that of the outer member. Therefore, unless the inner member is connected to the outer member, which can lead to stress fractures, complex support arrangements are generally required for the inner member.
It is thus desirable to provide a rotary furnace with a simple effective means for supporting an inner member or an outer member and a rotary furnace which is portable and can be loaded and unloaded in a quick and easy fashion. It is further desirable to provide a furnace that has a high thermal efficiency and has a relatively long, useful life through many thermal cycles.